(1) Field of the Invention
The present invention relates to a method of use of reversible polymeric surfactants or emulsifiers for forming colloidal gas aphrons (CGA) or colloidal liquid aphrons (CLA) and to the aphron compositions which are produced. The reversible CGA and CLA compare favorably with those made using conventional surfactants. The CGA and CLA are rapidly destabilized and coalesced by making small changes in the pH. The aphrons are used for transfer of chemicals to and from the aphrons. The reversible surfactants or emulsifiers are preferably nontoxic to microorganisms and suited for biotechnological applications involving living cells (e.g., fermentations). The CGA or CLA can particularly be used in a fluidized bed reactor.
(2) Description of Related Art
A. Separation Art
Many processes in the chemical, biochemical, and environmental industries involve phase-contacting operations in which one fluid phase is dispersed in a second immiscible fluid phase. The high interfacial area between the phases created by the dispersion allows rapid transfer of components from one phase to the other. Examples include gas-liquid and liquid-liquid reaction systems, solvent extraction, flotation, and gas scrubbing. The two phases can be dispersed mechanically, using equipment including stirred tanks, spray towers, and mixer-settlers. Surfactants or emulsifiers can also be used, both alone or in combination with mechanical methods, for generating and stabilizing dispersions. Once the transfer has taken place, it is often desirable to coalesce the dispersed phase and separate the two phases again.
Processes to treat radioactive, hazardous-chemical, and mixed wastes involve transfer of material from one phase to another. Such transfer operations are done to remove components of the waste for further characterization, to separate and/or concentrate particular waste fractions, or to add components needed to stabilize or remediate the wastes. Examples of such processes include extraction of organic wastes from aqueous liquids; stripping or absorption of gaseous components; flotation and foam fractionation for removal of colloidal materials, organic and metal ions from aqueous liquids; soil washing to eliminate hazardous wastes, and soil aeration to enhance bioremediation.
Often, the mass-transfer step is rate-limiting. In such instances, increasing the mass transfer rate directly increases the overall process efficiency. The rate of interphase mass transfer can be calculated as the product of three terms: the concentration difference (driving force) between the two phases, the mass-transfer coefficient, and the interfacial area per unit volume. In mass-transfer-limited systems, the concentration driving force is often small, because either the fluid from which the solute is transferring is dilute or the solubility of the transferring substance is low. The mass-transfer coefficient is weakly dependent on the chemical composition of the two phases and the local hydrodynamic conditions, and is difficult to adjust significantly. However, the interfacial area can vary by orders of magnitude. Extremely high interfacial areas can be produced by creating a fine dispersion of one phase in the other. Because the interfacial area per unit volume is inversely proportional to the average diameter of the dispersed phase, efforts to increase interphase mass-transfer rates generally focus on reduction of this diameter.
Mechanical agitation is often used for creating a dispersion of the phases. The shear forces generated through agitation overcome the surface tension and break up large droplets or bubbles into smaller ones. However, the resulting dispersion is thermodynamically unstable. To counteract the ongoing process of coalescence, the dispersion must be agitated continuously. Consequently, this approach is energy-intensive. It also does not scale up favorably. For similarly shaped mixing vessels, the power required to turn the impeller increases with the impeller diameter in the fifth power, whereas the reactor volume increases with the impeller diameter to the third power (McCabe et al., Unit Operations of Chemical Engineering, Fifth Ed. McGraw-Hill, New York, 350-357 (1993)).
Creation of a fine dispersion of one phase in the other can dramatically increase process efficiency. Colloidal gas aphrons (CGA) and colloidal liquid aphrons (CLA) have diameters ranging from submicron to over 100 microns and thus can provide exceedingly high interfacial areas for such applications (Matsushita et al., Colloids and Surfaces, 69:65-72 (1992)). Aphrons may have a surfactant-stabilized aqueous shell surrounding the gas or liquid droplet. Because surfactant molecules are amphipathic (i.e., they have both hydrophobic and hydrophilic regions), they accumulate at the interface between the phases and can reduce the surface tension. The surfactant molecules can generate an electric double layer that extends out from the interface and repels adjacent bubbles or droplets, thus stabilizing the dispersion against coalescence. This shell may also provide steric repulsion between adjacent aphrons, which stabilizes the aphrons against coalescence. CGA dispersions are easy and energy-efficient to produce, provide extremely high mass-transfer rates (Bredwell et al., Eighteenth Symposium on Biotechnology for Fuels and Chemicals, Gatlinburg, Tenn., May 5-9, (1996)), and can enhance the rate of bioremediation processes fed with synthesis gas (hydrogen and carbon monoxide), for instance. To date, the use of aphrons in commercial applications has been limited, because the stability imparted by the surfactant shell makes it difficult to break the dispersions and separate the phases following the phase-contacting step.
Sebba, F., Foams and Biliquid Foams-aphrons, John Wiley and Sons, New York, 62-71; 103-106 (1987) describes a CGA having colloidal dimensions on the order of 50 .mu.m in diameter, encapsulated by a soapy water "shell" and immersed in a continuous aqueous phase. The CLA is described as an analogue of a CGA with the gas replaced by a similarly sized droplet of a second liquid phase. The shell is thought to be relatively thick compared to surfactant layer surrounding conventional bubbles. Amiri and Woodburn, Trans. IChemE 68(a) 154-160 (1990) have estimated the shell thickness to be about 0.75 .mu.m for CGA. Both CGA and CLA dispersions are L:m. stable enough to be pumped with minimal coalescence from the device where they are formed to some remote site, where they are used. Extremely stable CLA suspensions have been stored in a stoppered bottle for years without evidence of deterioration. However, the degree of stabilization depends on the properties of the surfactant and the two phases.
The volumetric mass transfer rate (Q) is related to the mass-transfer coefficient (K), the interfacial surface area per unit volume (.alpha.), and the concentration driving force (.DELTA.C) according to the following equation EQU Q =K.alpha.(.DELTA.C)
Although K and .DELTA.C are often difficult to vary significantly, .alpha. is inversely proportional to the diameter of the dispersed phase. Thus, the small diameters of surfactant-stabilized aphron dispersions can yield extremely high Q values. It has been demonstrated that extremely high K.alpha. values (800 to 1800 h.sup.-1) could be obtained using CGA, even in an unagitated vessel (Bredwell et al., Eighteenth Symposium on Biotechnology for Fuels and Chemicals, Gatlinburg, Tenn., May 5-9 (1996).
Because they offer extremely high surface areas, CGA and CLA are being used increasingly to facilitate mass-transfer operations in chemical and environmental processes. CGA are used to supply oxygen to biological waste-treatment bioreactors. Other proposed applications for CGA include providing gaseous substrates for synthesis-gas fermentations (Bredwell, et al., Eighteenth Symposium on Biotechnology for Fuels and Chemicals, Gatlinburg, Tenn., May 5-9 (1996)) flotation of sulfur crystals from catalytic processing of natural gas (Amiri and Woodburn, Trans. IChemE 68(a), 154-160 (1990)), and soil washing (Roy et al., Separation Science and Technology 27(12) 1555-1568 (1992)). Proposed applications for CLA include extraction of enzymes from fermentation broth (Save et al., Biotechnology and Bioengineering 41:72-78 (1993)), immobilized-enzyme bioreactors (Lye et al., Biotechnology and Bioengineering 51:69-78 (1996)), stripping dilute solutes from aqueous phases and delivering apolar substrates to aqueous reaction systems.
A key problem which limits the applications for CGA and CLA is that the stability imparted by the surfactant hinders phase separation after the mass-transfer step is complete. In some cases, a destabilizing agent can be added to solubilize and break the dispersion. However, this approach adds complexity and cost to the process, and may necessitate additional separation steps to remove the destabilizing agent. Hence, there is a need for new methods to produce CGA and CLA that remain stable during the mass-transfer step, but afterwards can be rapidly coalesced.
The patent art has described a number of methods for forming microbubbles (polyaphrons) using a surface active agent (surfactant) in an aqueous liquid. Typically the microbubbles contain up to 65% gas, usually between 20% to 60% gas, as described by Sebba (U.S. Pat. No. 3,900,420). The half lives are 5 to 10 minutes for the soaps used by Sebba. The diameter of the microbubbles was between about 1 to 10 microns. Sebba (U.S. Pat. No. 4,486,333) describes biliquid polyaphrons using various surfactants by first forming gas polyaphrons and then adding a non-polar liquid. Michelsen and Sebba (U.S. Pat. No. 5,314,644) describe a particular apparatus for forming the microbubbles. Yoon et al (U.S. Pat. No. 4,981,582; fine particle flotation); Lu et al (U.S. Pat. No. 5,443,985; cell culture) and Schutt et al (U.S. Pat. No. 5,605,673; contrast agents). Other patents of general interest are U.S. Pat. Nos. 3,891,571 to Lambou et al; 4,684,479 to D'Arrigo et al; 4,668,632 to Young et al; 5,009,792 to Pettersen and 5,223,429 to Tepic relating to foams and their uses.
B. Polymer Art
There are a number of patents relating to reversible surfactants for a variety of non-microbubble applications. Some are reversible as a function of pH. U.S. Pat. No. 3,950,296 to Kangas et al describe surfactants used for coacervation (precipitation) of an ingredient in an aqueous solution. U.S. Pat. No. 4,735,731 to Rose et al describes pH sensitive thickening surfactants.
Polymeric emulsifiers and thickeners, which are surfactants, are well known. Polymeric emulsifiers have been used to stabilize oil droplets in water for a variety of applications including, but not limited to, aqueous cleaning operations, suspension polymerization, food applications, cosmetics, pharmacy, agriculture and bitumen processing. The emulsifiers generally contain both hydrophilic and hydrophobic groups, giving the polymer an "amphipathic" character. Examples of such polymeric emulsifiers and their properties are described in standard texts such as Irja Pirma, Polymeric Surfactants, Marcel Dekker, (1992). This text summarizes an extensive body of literature regarding hydrophilic/hydrophobic diblock, triblock, graft and random copolymers. Specific examples of polymeric emulsifiers are provided in Great Britain Patent GB 2,115,002A to Baker (1983), which discloses block or graft copolymers of hydrophobic monomers with hydrophilic monomers. U.S. Pat. No. 5,021,526 to Ball, describes random terpolymer emulsifiers made from water-soluble vinyl monomers, water-insoluble vinyl monomers containing 12 to 30 carbon atoms, and polymerizable acid monomers. In addition, hydrophobically functionalized crosslinked polyacrylic acid may be used as polymeric emulsifiers, as reported by R. Y. Lochhead ACS Symposium Series, Vol. 462, 101 (1991). In all of these examples, the molecular structure of the emulsifier contains both hydrophobic and hydrophilic groups to achieve an amphipathic nature. In general, the hydrophilic groups may be anionic, cationic or nonionic in nature. Furthermore, although there has been very limited discussion of polymeric emulsifiers which allow the emulsion to be reversibly formed and broken, as discussed in Great Britain Patent GB 2,006,744A to Sonnerqard (1979), no such emulsifiers which are truly effective have yet been obtained.
Alkali swellable or alkali soluble thickeners (AST's) have found wide application for thickening paint, coating, textile, consumer product, and adhesive formulations. In general, these thickeners exhibit low viscosity under acidic conditions, and high viscosity under basic conditions making them easy to manufacture and blend into formulations while providing excellent thickening properties in the final formulation upon pH adjustment. As described in several reviews and monographs (G. D. Shay, Advances in Chemistry, 223, 457 (1989) and Toshio Murakami, R. H. Fernando, J. E. Glass, Surface Coating International, 76, 8 (1993)), these thickeners are typically produced by emulsion polymerizations of acrylic or methacrylic acid with a hydrophobic monomer such as ethyl acrylate. The acid groups may be positioned on the surface of the resulting beads by semi-batch or multi-stage addition of the hydrophilic monomer in the latter stages of an emulsion polymerization such as described in U.S. Pat. Nos. 5,266,646 and 5,451,641 to Eisenhart. Generally, such polymers contain more than 40 wt. % of the hydrophobic monomer. For example, Murakami et al., teach compositions containing 48-51 wt. % of the hydrophobic monomer, Toshio Murakami, R. H. Fernando, J. E. Glass, Surface Coating International, 76, 8 (1993). Rodriguez and Wolfe report systems containing 66 wt. % of the hydrophobic ethyl acrylate monomer and 33 wt. % of the hydrophilic methylmethacrylate monomer, Macromolecules, 27, 6642 (1994). U.S. Pat. No. 4,351,754 to Duprey, teaches that at least 30% alkyl methacrylates with one to four carbon atoms must be used. U.S. Pat. No. 4,801,671 to Shay, teaches compositions with 15% to 50% carboxy monomer, 10% surfactant monomer, and the balance being a hydrophobic ethyl acrylate monomer. When utilizing high amounts of hydrophilic acid monomer, it is often necessary to include crosslinking agents to maintain insolubility and high thickening efficiency, Toshio Murakami, R. H. Fernando, J. E. Glass, Surface Coating International, 76, 8 (1993).
Water-insolubility of the emulsifiers and thickeners described above is provided via hydrophobic comonomers or functional groups such as aliphatic esters of acrylic or methacrylic acid. Polymeric emulsifiers also contain hydrophobic comonomers to impart compatibility with the emulsified oil phase, while thickeners contain hydrophobic comonomers to provide water insolubility at acidic pH. These classes of materials have never previously been made by aqueous emulsion or suspension polymerization without hydrophobic comonomers or hydrophobic functional groups.
Polymers which are sensitive to pH have been widely used in pharmaceutical and agricultural controlled-release formulations, see for example, Advances in Polymer Science, Vol. 122 Springer Verlag Berlin Heidelberg, 1995. In particular, such polymers have been used as coatings, and in particular, as enteric coatings. Typically, copolymers of hydrophilic carboxy or amine functional monomers with hydrophobic water-insoluble monomers are described for such coatings. These materials provide water-insoluble coatings at one pH, thereby preventing drug diffusion and release. At a second pH, the coatings become water-permeable or water-soluble resulting in drug delivery. For example, medications are commonly coated with enteric coatings which are water-insoluble at the acidic pH of the stomach and water-soluble in the basic pH of the intestine.
U.S. patent application Ser. No. 08/695,237, filed Aug. 8, 1996 now U.S. Pat. No. 5,739,210, describes a new class of reversible block/graft copolymer surfactants that are inexpensively produced from polyethylene glycol (PEG) and polymethacrylic acid (PMAA) by free-radical polymerization, which disclosure is incorporated herein by reference. These copolymers consist of a polymeric backbone with oligomeric grafts extending linearly from the backbone and are used for purposes unrelated to aphrons. The graft side chains are capable of forming hydrogen-bonded complexes with the backbone under acidic conditions to form blocks of hydrophilic and relatively hydrophobic polymer. The complex is hydrophobic because the hydrophilic acid and ether moieties are associated with one another, and are therefore unavailable to interact with the solvent; these hydrophilic groups are effectively buried in the complex, and only the hydrophobic methyl and ethylene groups are available to interact with the solvent. The copolymers assume a multiblock architecture with alternating hydrophilic and hydrophobic blocks under complex-promoting, acidic conditions, and revert back to a hydrophilic graft copolymer when the complex is broken (basic conditions). There is a need for a method and compositions which provide reversible aphrons.
OBJECTS
It is an object of the present invention to provide compositions which are a fine dispersion of a fluid phase in a second, immiscible phase using CGA or CLA so that the stability properties of the dispersion can be controlled by manipulating a change of environmental condition of the aphrons, particularly pH. In this way, the dispersion of CGA or CLA can be maintained stable during the phase-contacting step and then quickly coalesced for phase separation. It is further an object of the present invention to provide a method of producing the dispersions of CGA and CLA, maintaining them stable for a desired period of time, and then coalescing them. It is further an object to provide the dispersions by contacting the immiscible fluid phases, together with a novel, reversible, polymeric surfactant, in a high shear zone wherein dispersions are coalesced by making a small, predetermined change in pH. These and other objects will become increasingly apparent by reference to the fop owing description and the drawings.